Evaluation of the nitrate-based colorimetric method for testing the susceptibility of Mycobacterium tuberculosis to streptomycin and ethambutol in liquid cultures.

OBJECTIVES To evaluate the inexpensive colorimetric nitrate reductase-based antibiotic susceptibility (CONRAS) assay for testing the susceptibility of Mycobacterium tuberculosis to streptomycin and ethambutol in liquid cultures, and to compare the CONRAS test with the manual mycobacteria growth indicator tube (MGIT) test, using the radiometric BACTEC 460TB method as reference. METHODS A total of 89 M. tuberculosis isolates were tested for susceptibility to streptomycin and ethambutol using the CONRAS and manual MGIT methods and the results were compared with BACTEC 460TB. Isolates with discrepant results between the CONRAS test and BACTEC 460TB were analysed using the agar proportion method, Etest and mutation analysis of genes involved in resistance to streptomycin and ethambutol. RESULTS The agreement between the CONRAS test and BACTEC 460TB was 88% for streptomycin and 84% for ethambutol. The corresponding agreement of the manual MGIT test with BACTEC 460TB was 89 and 80%, respectively. There was good agreement for streptomycin and moderate agreement for ethambutol between the CONRAS and manual MGIT tests on one hand and BACTEC 460TB on the other (CONRAS test, kappa(streptomycin) 0.74 and kappa(ethambutol) 0.59, P < 0.001; manual MGIT test, kappa(streptomycin) 0.77 and kappa(ethambutol) 0.50, P < 0.001). CONCLUSIONS There is good agreement for the two non-radiometric liquid culture methods (CONRAS and manual MGIT) compared with BACTEC 460TB for the detection of streptomycin resistance. Further standardization is needed for testing of ethambutol resistance using the CONRAS and manual MGIT assays.